1. Field of the Invention
The present invention relates to a data transmitter-receiver for performing bidirectional data transmission in a communication environment in which various states are dynamically changed and a transmission path having a delay, bidirectional data transmitting system, and data transmitting-receiving method.
2. Description of the Related Art
When performing bidirectional communication in a transmission path having a delay, there may be an inconsistency between the state of a receiver when a transmitting-side apparatus generates transmission data to be transmitted to a receiving-side apparatus and the state of a receiver when the receiving-side apparatus receives transmission data. Therefore, a mechanism for compensating this inconsistency is necessary. When the transmitting-side apparatus and receiving-side apparatus perform execution processing in harmony because of load dispersion or the like, it is necessary to grasp mutual states each other by, for example, synchronizing them.
As an example, a case of stereophonic transmission is described below.
A system for realizing stereophony by a two-channel stereophonic system is studied in recent years and a technique for using head transfer functions which are transfer function from sound sources to ears of a listener is known. When input sound sources are filtered by HRTFs digitally and the filtered signals are played back through two-channel loudspeakers or headphones, the listener perceives stereophonic sound effect. This is a technique suited for reproducing stereophonic sound in a cellular terminal compared to a multichannel reproducing system for reproducing surround acoustic space by arranging many speakers around. However, the head transfer function changes depending on the relative position between a listener and a sound source and direction of the listener. To reproduce a dynamic sound field in which self or other party for speaking, that is, the sound source or listener optionally moves, it is necessary to follow the position of the sound source and the listener and direction of the head. Therefore, a method for detecting position and rotation of the head and sequentially updating head transfer function filters is used (for example, refer to “Reassessment of the role of head movements in human sound localization”, F. Wightman, et al., J. Acoust. Soc. Am., 95(5), pp. 3003-3004; hereinafter referred to as Non-patent Document 1). By introducing the following function, the improvement of the accuracy of directional perception is expected.
When reproducing stereophony by the above technique in mobile communication, there is a terminal, as shown in FIG. 12, for receiving a plurality of sound-source data and performing stereophonic processing. However, in the case of this terminal-side processing system, large bandwidth is also required for transmitting all acoustic data relative to sound sources assigned in a stereophonic space. When it applies to a terminal having restrictions on processing power and transmission bandwidth such as a cellular phone, it is preferable to minimize them.
As another method, it is considered to use a stereophonic transmitting system comprising a server for receiving a plurality of sound sources data and transmitting the sound data to which stereophonic processing has been applied to a terminal as shown in FIG. 13 and a terminal for reproducing the received stereophonic data. In the case of this server-side processing system, problems of the above-described terminal throughput and transmission data quantity are settled. However, the position and direction of the listener's head must be transmitted to a server in order to perform stereophonic processing according to the movement of the listener. This causes another problem when a large transmission delay between the server and terminal exists. That is a mismatch between the received stereophonic data and actual position and direction of the head (head tracking delay).
Moreover, as still another method, it is considered to use a stereophonic transmitting system comprising a server for receiving a plurality of sound-source data and transmitting the data to which stereophonic processing has been applied to a terminal and a terminal for correcting and reproducing received stereophonic data received in accordance with the latest positional and directional information as shown in FIG. 14 (for example, refer to U.S. Pat. No. 6,259,795; hereinafter referred to as Patent Document 1). In the case of this technique, as disclosed in Patent Document 1, when a sound field is static, it is possible to generate a stereophonic field with a simple operation and a reduced-tracking delay at the terminal side if the server sends stereophonic-processed sound to the terminal and compensation of the acoustic image mismatch due to the small movement of the listener's head.